The invention relates to a brake for a hydraulic vehicle brake system according to the pre-characterizing clause of Claim 1, to a sealing ring for such a brake, to a method for producing such a sealing ring and to a method for coating a brake piston.
Hydraulic vehicle disk brakes have at least one piston/cylinder unit, by means of which an associated brake lining, which consists of a lining carrier plate and of a friction lining fastened thereto, can be pressed against the brake disk, in order to brake the rotational movement of a wheel connected to the brake disk. The brake lining is pressed onto the brake disk as a result of the piston being displaced in the direction of the brake disk by means of hydraulic pressure. So that this action functions perfectly, the piston must be sealed off relative to the bore or cylinder, respectively, in which said piston is received. This fluid sealing is conventionally achieved by means of an annular elastomeric seal which is normally held in a groove of the bore or cylinder, respectively, and the radially inner circumferential surface of which bears on the outer surface of the piston. The elastomeric sealing ring mostly has a square or rectangular cross section.
During braking, the piston moves in the direction of the brake disk by means of hydraulic pressure and the elastomeric sealing ring is somewhat taken up by the displacing piston and, as a result of this and of the hydraulic pressure, is somewhat deformed. When the brake is released, that is to say when the hydraulic pressure ceases, the deformed elastomeric sealing ring exerts a return force on the piston and entrains the latter a little distance away from the brake disk. This behavior is plainly desirable, since it resets the brake piston and ensures that, after braking has ended, a so-called lifting clearance is established between the friction lining and the brake disk. Reference is made, in this respect, to German Patent 1,600,008 which describes this behavior in detail. In order to assist the above-described behavior of the elastomeric sealing ring, often even the groove side wall which is located on the brake-lining side and in which the sealing ring is held is slanted, that is to say the groove cross section widens from the groove bottom (see, in this respect, French Patent 1,504,679). So that the sealing ring can reset the brake piston, some adhesion between the cooperating surfaces of the sealing ring and of the brake piston is necessary. If the sliding properties between these surfaces were too good, the piston would be prevented from being reset, since it would be slideably displaced relative to the sealing ring almost immediately, with the result that there would no longer be any deformation of the sealing ring and, consequently, also any resetting force.
On the other hand, under certain circumstances, it is highly desirable for the brake piston to slide relative to the sealing ring. This is the case, for example, when the brake is being assembled. Here, after the sealing ring has been inserted into the cylinder wall groove receiving it, the brake piston has to be pushed into the cylinder. If the piston and sealing ring were unable to slide relative to one another, it would be highly likely that the sealing ring would be twisted in the groove or that it would be damaged and partly sheared off. In certain operating states of the brake, too, however, it is, necessary for the brake piston to slide relative to the sealing ring, for example when the thickness of the friction lining decreases due to wear, so that the brake piston has to be displaced correspondingly further in the direction of the brake disk in order for the friction lining to abut the brake disk. Under such circumstances, the deformability of the elastomeric sealing ring is exceeded, and the piston has to slide relative to the sealing ring in order to assume a new position relative to the latter. Sliding of the brake piston relative to the sealing ring may become necessary even during very sharp braking operations, since then, due to the high forces, the brake housing is widened, the material of the friction lining is compressed, etc., so that a correspondingly longer displacement travel of the brake piston has to be produced.
In order to promote the sliding properties between a brake piston and the elastomeric sealing ring sealing it off, it has long been conventional to soak the sealing ring in an assembling fluid prior to assembly. Sealing rings soaked in this way are inserted into the groove in the cylinder wall of the piston/cylinder unit, and the associated piston can then be pushed readily into the cylinder, without damaging the sealing ring.
However, the use of an assembling fluid also leads to a series of problems. Thus, it is necessary to make sure that the sealing rings do not dwell too long in the assembling fluid. Also, constituents of the sealing rings are dissolved in the assembling fluid, so that the latter has to be changed from time to time. Furthermore, the sealing rings soaked in the assembling fluid must be further processed, that is to say assembled, within a specific period of time. Moreover, when a vehicle brake system is filled with hydraulic fluid for the first time, the assembling fluid may lead to undesirable secondary effects, such as, for example, foaming.